The anticipated advantages of arc-discharge automotive headlamps are an improved roadway illumination pattern by comparison with conventional filament headlamps; lower and shorter headlamp reflectors which permit improvements in aerodynamic efficiency and greater freedom in automotive styling, e.g. lowering the hood line; longer-lived lamps lasting the life of the vehicle; and lower consumption of energy which saves fuel and also reduces the thermal load on the plastic components of the headlamp housing.
An essential requirement of an automotive headlamp is ability to provide light immediately when needed, including instant relight after a momentary turn-off. U.S. patent application Ser. No. 157,436 filed Feb. 18, 1988 by Rolf S. Bergman et al., titled Xenon-Metal Halide Lamp Particularly Suited for Automotive Applications, abandoned in favor of continuation Ser. No. 07-539,276 filed Jun. 18, 1990, assigned to the same assignee as the present invention and incorporated herein by reference, discloses a xenon-metal halide discharge lamp which combines the high efficacy and long life of the metal halide lamp with the instant-light capability of a xenon arc tube.
But the presence of high pressures of xenon in a xenon-metal halide lamp aggravates gravity-induced convention, and this tends to limit the benefits achieved. Convection in the fill causes undesirable results: (1), upward bowing of a horizontal arc, while lamp optics prefer a straight line; (2), higher temperature of the hot spot at the envelope wall above the arc, a condition which tends to shorten lamp life; and (3), lower temperature of the cold spot at the wall under the arc, a condition which reduces the metal halide vapor pressure and results in lowered lamp efficacy.
U.S. patent application Ser. No. 320,736 filed Mar. 8, 1989 by Gary R. Allen et al. now abandoned, and its continuation in part application Ser. No. 07-579,129 filed Sep. 6, 1990, titled Acoustic Resonance Operation of Xenon Metal Halide Lamps and assigned to the same assignee as the present invention, are incorporated herein by reference. The continuation in part application, which for brevity will henceforth be referred to herein as Allen et al, teaches that gravity-induced arc bowing may be reduced or eliminated by operating the lamp on current having AC components at frequencies in preferred bands within the range extending from about 5 kHz to 1 MHz in order to excite desired acoustic modes within the lamp fill. Preferred bands are those in which acoustic perturbations compel gas or vapor movement patterns in the fill which counter gravity-induced convection. The straightening of an arc in this fashion by excitation of selected acoustic modes is termed acoustic straightening, and such manner of operation, acoustic resonance operation.
In acoustic resonance operation, frequency bands that drive the arc asymmetrically toward the wall, or cause unstable movement of the arc or of the plume or luminous aureole surrounding the arc, are to be avoided. The bands or windows in which arc-straightening occurs are relatively narrow, for instance a maximum bandwidth of 10% of a ballast output frequency of 40 kHz. The imposition of a suitable frequency modulation on the ballast frequency can greatly broaden the width of the windows for arc-straightened stable operation, for instance to 30% of the center frequency.
In Allen et al, a ballast circuit is described which is energized from the usual 12 volt DC automobile storage battery supply. It comprises an input stage in the form of a DC-to-DC boost converter which is controlled by feedback from the lamp, and a DC-to-AC inverter for the output stage. Lamp voltage and current are fed back to the input converter to regulate lamp power, and the output stage may be frequency-modulated to broaden the frequency window for stable arc-straightened operation.
The acoustically-straightened arc discharge headlamp as proposed in Allen et al, is far superior to conventional incandescent headlamps as regards lifetime, light output, packaging size, and styling freedom for automotive applications. But it has the disadvantage of higher cost, due primarily to the ballast and starter circuits required for instantaneous starting and running of the discharge headlamp.
A reduction in size and cost could be achieved by connecting the ballast circuit directly to the automobile's DC battery source and eliminating the DC/AC inverter. But, as discussed in application Ser. No. 157,360 filed Feb. 18, 1988 by Richard L. Hansler et al, titled Metal Halide Lamp Having Vacuum Shroud for Reducing Cataphoretic Effects, assigned to the same assignee as the present invention and incorporated herein by reference, now U.S. Pat. No. 4,935,668, metal halide-containing lamps operated on DC tend to cataphoresis wherein the metal ion, e.g. sodium, of the dissociated metal halide collects in the cathode end region of the lamp. The sodium contribution to light generation is thereby reduced, and the effect is most noticeable in lamps operated horizontally as in auto headlamps.